# std::hash<Key>::operator()

From cppreference.com
< cpp‎ | utility‎ | hash

Specializations of std::hash should define an `operator()` that:

- Takes a single argument key of type `Key`.
- Returns a value of type std::size_t that represents the hash value of key.
- For two parameters `k1` and `k2` that are equal, std::hash<Key>()(k1) == std::hash<Key>()(k2).
- For two different parameters `k1` and `k2` that are not equal, the probability that std::hash<Key>()(k1) == std::hash<Key>()(k2) should be very small, approaching 1.0 / std::numeric_limits<size_t>::max().

### Parameters

key | - | the object to be hashed

### Return value

A std::size_t representing the hash value.

### Exceptions

Hash functions should not throw exceptions.

### Example

The following code shows how to specialize the std::hash template for a custom class. The hash function uses Fowler–Noll–Vo hash algorithm.

Run this code

    #include <cstdint>
    #include <functional>
    #include <iostream>
    #include <string>

    struct Employee
    {
        std::string name;
        std::uint64_t ID;
    };

    namespace std
    {
        template <>
        class hash<Employee>
        {
        public:
            std::uint64_t operator()(const Employee& employee) const
            {
                 // computes the hash of an employee using a variant
                 // of the Fowler-Noll-Vo hash function
                 constexpr std::uint64_t prime{0x100000001B3};
                 std::uint64_t result{0xcbf29ce484222325};

                 for (std::uint64_t i{}, ie = employee.name.size(); i != ie; ++i)
                     result = (result * prime) ^ employee.name[i];

                 return result ^ (employee.ID << 1);
             }
        };
    }

    int main()
    {
        Employee employee;
        employee.name = "Zaphod Beeblebrox";
        employee.ID = 42;

        std::hash<Employee> hash_fn;
        std::cout << hash_fn(employee) << '\n';
    }

Output:

    12615575401975788567

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